1. Field of the Invention
The present invention generally relates to multipiece molds, and specifically relates to guide mechanisms for ensuring precise placement of mold parts when in the mold forming position.
2. Brief Discussion of the Prior Art
Multipiece molds for forming molded products, and in particular injection blow molded products, are well known. Generally, a parison comprising the material to be molded and located on the end of a blow molding tube is inserted into position in a lower mold half. The upper mold half is lowered into the mold forming position and high pressure air and/or movement of the air tube results in the parison being expanded into all portions of the mold. After suitable cooling, the mold halves are separated and the blow molded product removed.
In many blow molded products, there are recessed bottoms on the molded product which make it difficult, if not impossible, to separate the mold halves without damage to the molded part. In such molds, it is generally acceptable to utilize a third mold part identified as a bottom end plug. This plug is slid into the mold forming position and, after the blow molding is complete, can be retracted so that any protrusion on the end plug (used to form the inner curvature to the bottom of the product) does not obstruct removal of or otherwise damage the product from the upper and lower mold portions.
In order to form a properly molded article, it is necessary that the edges of each of the mold portions fit precisely. Wear between mold parts would result in undesirable part lines being formed on the moulded product. Generally, the upper and lower mold halves are moved on bearinged and lubricated posts relative to one another, thereby maintaining a high degree of tolerance with respect to these mold halves and the direct and non-sliding contact between the upper and lower mold halves accounts for the general absence of wear.
However, where a bottom end plug is required, the plug is slid into and out of the mold forming position, and thus there can be sliding contact with one or both of the mold halves. Positioning of the end plug is generally accomplished by using an end plug mount which moves on lubricated sliding guide surfaces. Because of the industrial setting in which such molds are operated, the lubricated surfaces become contaminated and wear of the bottom end plug mount sliding surfaces occurs. In as few as 10 to 20 thousand mold cycles, the bottom end plug mount sliding surfaces can wear enough that the bottom end plug does not precisely fit in the mold forming position with resultant mold part lines being created in the bottom of the blow-molded product. Even if the sliding surfaces are protected by seals and/or protective boots, unacceptable wear occurs after 40 to 60 thousand cycles.
When the above described wear problem occurs, the bottom end plug, the mounting block and/or the upper and lower mold halves must be refurbished by the addition of any metal worn away and then refinishing to their original tolerances. Because refurbishment is extremely expensive, in many instances it is more cost-effective to simply replace the upper and lower mold halves along with the bottom end plug.
Thus, it is desirable to provide a mold guide structure which will precisely and accurately position moveable portions of a mold in the mold forming position and at the same time allow it to easily move into and out of the mold forming position through a large number of operating cycles.